The present invention relates to processes and apparatus for the purification of carbon dioxide. In particular, the invention relates to processes and apparatus for the removal of at least one “heavy” impurity from crude carbon dioxide by mass transfer separation at sub-ambient temperatures and super-atmospheric pressures. The invention has particular application to the purification of crude carbon dioxide comprising significant amounts of at least one “light” impurity.
By “light” impurity, the Inventors are referring to an impurity that is more volatile than carbon dioxide. Examples of “light” impurities include nitrogen (N2), oxygen (O2), argon (Ar), hydrogen (H2), helium (He); methane (CH4); carbon monoxide (CO), neon (Ne), xenon (Xe), krypton (Kr), nitric oxide (NO) and nitrous oxide (N2O).
By “heavy” impurity, the Inventors are referring to an impurity that is less volatile than carbon dioxide. Examples of “heavy” impurities include hydrogen sulfide (H2S); methanol (MeOH); C3-C8 hydrocarbons such as propane; carbon disulfide (CS2); carbon oxysulfide (COS); dimethyl sulfide (Me2S) and other organic sulfur compounds; nitrogen dioxide (NO2); sulfur dioxide (SO2); sulfur trioxide (SO3); and ammonia (NH3).
C2 hydrocarbons such as ethane, ethylene and acetylene form azotropic mixtures with carbon dioxide so they can behave as “light” impurities or “heavy” impurities depending on concentration.
Carbon dioxide from naturally occurring carbon dioxide sources, such as natural carbon dioxide fields and natural gas deposits, is used for enhanced oil recovery (EOR) in some areas of the world. Some of these sources contain hydrogen sulfide, which is undesirable for pipeline transport since hydrogen sulfide is toxic and corrosive in the presence of water. In addition, it is not desirable to introduce hydrogen sulfide to the crude oil that is being extracted by the EOR process.
Processes for the removal of hydrogen sulfide from carbon dioxide are known. For example, U.S. Pat. No. 3,417,572A (Pryor, 1968) discloses a method of treating hydrogen-rich gas comprising carbon dioxide and hydrogen sulfide. The hydrogen sulfide and carbon dioxide are condensed and separated from the hydrogen-rich gas. The condensed gases are then fed to a distillation column for separation into an essentially hydrogen sulfide-free carbon dioxide overhead vapor and a bottoms liquid containing at least 10 vol. % hydrogen sulfide. The separated hydrogen-rich gas is scrubbed to remove any residual carbon dioxide and hydrogen sulfide which is then also fed to the distillation column. Overhead vapor is condensed using an external closed cycle of propane refrigerant and bottoms liquid is re-boiled using process cooling water. The distillation column has 100 trays and operates at about 590 psia (˜41 bar) so that the overhead temperature is 42° F. (˜6° C.) and the bottom temperature is about 45° F. (˜7° C.).
U.S. Pat. No. 3,643,451 A (Foucar, 1972) discloses a method of producing high purity, high pressure carbon dioxide from a concentrated low pressure mixture of acid gases. The gaseous mixture is compressed, cooled and condensed and fed to a distillation column where it is separated into a high purity (at least 99.95%) carbon dioxide overhead vapor and a bottoms liquid containing condensed sulfur-containing gases. The overhead vapor is condensed using an external closed cycle of ammonia refrigerant and refrigeration duty for cooling and condensing the feed is provided by vaporizing bottoms liquid, carbon dioxide overhead liquid and the external refrigerant. The distillation column system operates at about 300 to 350 psia (˜21 to 24 bar) so that the overhead temperature is −5 to −10° F. (˜−21 to −24° C.) and the bottoms temperature is 40 to 70° F. (˜5 to 21° C.). A bottoms product of 97% hydrogen sulfide is produced in the example.
WO81/02291A (Schuftan, 1981) discloses a method for separating a gas mixture comprising carbon dioxide, at least one gas having a lower boiling point than carbon dioxide and at least one impurity (typically hydrogen sulfide) having a higher boiling point than carbon dioxide. The gas mixture is cooled and distilled in a first column to a product gas free of the impurity and a liquid fraction containing the impurity. Pure carbon dioxide is obtained in a second distillation column, which operates slightly above the triple point pressure (˜518 kPa) of carbon dioxide. Liquid product from the first column is flashed at an intermediate pressure to remove dissolved light impurities, then further reduced in pressure and evaporated before being fed to the second column as vapor. The carbon dioxide overhead vapor is practically free of impurities and the bottoms liquid fraction is rich in impurities, typically containing sulfur compounds (primarily hydrogen sulfide) at a plurality of up to 50 vol. %. Reflux and re-boil are effected by a heat pump cycle which uses purified carbon dioxide as the working fluid. The working fluid is passed through a compressor, a heat exchanger and a re-boiler immersed in the bottoms liquid, where it is condensed before being fed back to the top of the column as reflux. A substantially pure carbon dioxide product is withdrawn from the circulating carbon dioxide immediately upstream of the compressor at a pressure of about 5 atm. and at near-ambient temperature.
The Inventors have also developed a process for the removal of “heavy” impurities such as hydrogen sulfide from crude carbon dioxide. The process is described in co-pending U.S. patent application Ser. No. 13/456,854 filed on 26 Apr. 2012, the disclosure of which is incorporated herein by reference. In that process, “heavy” impurities are removed from crude carbon dioxide by sub-ambient temperature distillation of crude carbon dioxide in a distillation column system operating at superatmospheric pressure(s) to produce carbon dioxide-enriched overhead vapor and a bottoms liquid enriched with the “heavy” impurities. The Inventors discovered that, where such processes involve at least one heat pump cycle using as working fluid carbon dioxide-containing fluid from the distillation system, significant savings in power consumption are realized when the process uses more than one recycle pressure in the heat pump cycles(s).
In addition to the “heavy” impurities, crude carbon dioxide can also contain significant quantities of “light” impurities. The “light” impurities tend to concentrate in the carbon dioxide product. Thus, depending on the purity specification of the carbon dioxide product, it may be necessary to also remove these “light” impurities from the carbon dioxide. Most conventional processes remove the “light” impurities from the carbon dioxide product. However, U.S. Pat. No. 3,417,572A and WO81/02291A (discussed above) are examples of prior art references that disclose processes for removing the “light” impurities before the “heavy” impurities.
GB971362 (Ruhemann, 1964) discloses a process for the removal of both “light” and “heavy” impurities from natural sources of carbon dioxide. Crude carbon dioxide feedstock at 30° C. and 110 atm is cooled and condensed moisture removed. The gas is then dried, and cooled by indirect heat exchange to form partially condensed feedstock which is expanded to 20 atm and then fed to the lower column of a double fractionating column where it is separated into “light” impurity-enriched overhead and liquid carbon dioxide containing the “heavy” impurities. The overhead is removed, expanded and removed from the process. The liquid carbon dioxide is expanded to 8 atm and fed to the upper column of the double column where it is separated to produce carbon dioxide overhead vapor and “heavy” impurities-enriched bottoms liquid. The carbon dioxide gas is condensed and removed as liquid product, and the bottoms liquid containing the “heavy” impurities is expanded and removed from the process with the “light” impurities.